******************************************************************************* Information about the OGLE project is also accessible via World Wide Web (WWW): "http://www.astrouw.edu.pl" ******************************************************************************* THE OPTICAL GRAVITATIONAL LENSING EXPERIMENT. The Optical Gravitational Lensing Experiment (OGLE) is a long term observational program with the main goal to search for dark, unseen matter using the microlensing phenomena. The idea of employing microlensing for that purpose was originally proposed by Paczynski (ApJ, 304, 1; ApJ Letters, 371, L63). The Magellanic Clouds and the Galactic Bulge are the most natural locations to conduct such search due to a large number of background stars that are potential targets for microlensing. The LMC and SMC stars may be lensed mostly by the Galactic halo objects, in case of the Galactic Bulge stars one expects an additional component -- microlensing by low-mass disk stars. In both cases the optical depth for microlensing is very small -- about 10^{-6}. Therefore a massive, long term survey must be conducted to a) detect and b) collect statistatically significant sample of microlensing events to draw any conclusion about the nature of dark matter. The OGLE project started in 1992 and the Galactic Bulge has been selected as the first target. Observations are carried out at the Las Campanas Observatory, Chile which is operated by the Carnegie Institution of Washington. So far nine microlensing events have been found in the direction of the Galactic Bulge and possibly one event caused by a binary lens. The table summarizes parameters of the events: Event T_max t_0 Amplifi- I mag V - I remarks JD-2448000 (days) cation (min light) OGLE #1 1153.9 25.9 2.7 18.8 1.5 OGLE #2 803.4 45.0 6.5 19.1 1.6 OGLE #3 831.5 10.7 1.26 15.89 1.7 OGLE #4 807.3 14.0 5.8 19.3 1.5 OGLE #5 824.3 12.4 11.5 17.9 1.6 OGLE #6 818.9 8.4 6.9 18.1 1.5 OGLE #7 1172.5 80 - 17.5 1.9 (binary lens) OGLE #8 1218.6 49.5 1.9 17.8 1.7 OGLE #9 816.6 18.7 1.7 19.2 1.8 OGLE #10 801.4 61.6 1.10 15.82 2.1 OGLE #11 1537.5 12.0 1.31 18.20 1.6 (first real time lens) OGLE #12 1582.3 20.9 2.03 18.73 2.2 In 1994 observing season a new feature is implemented in the OGLE data system: the "Early Warning System" (EWS). It allows the detection and identification of microlensing events in real time. Thus, the system makes it possible to reschedule observations to obtain the best coverage of a microlensing event while it unfolds, as well as to arrange follow-up photometric and/or spectoscopic observations from other observatories. See details in "early.warning" file in this directory. First ever ongoing microlensing event was caught on July 8, 1994. The EWS detected a microlens candidate in Baade's Window field BW6. The event was low-magnification (0.3 mag) short-lasting (t0=12 days) event. First OGLE ALERT info has been sent to interested astronomers for follow-up observations. The finding chart and light curve of OGLE #11 microlens can be found in /ogle/ews/bw6 directory. Second ongoing event was detected on August 20, 1994. The star from Baade's Window, BW5 I 83758, underwent microlensing. The time scale was about 20 days. The finding chart and regularly updated light curve of OGLE #12 candidate can be found in /ogle/ews/bw5 directory. In 1995 season 6 more events, that is OGLE 14 - 19, have been discovered so far with the EWS system (beginning of August). * * * The huge amount of the collected photometric data necessary for microlensing search provides also an unique observing material for conducting independent side projects. Results of the first of them on the structure of our Galaxy have already been published. The huge catalog of periodic variable stars is available in directory /ogle/var_catalog. The observing time when the Galactic Bulge is not visible or seeing is too large for observations of the dense Bulge fields is used for additional observational side projects. In 1992 clusters of galaxies were observed for supernovae, in 1993 globular clusters and Sculptor dwarf galaxy were observed for variable stars. Another project being pursued when the bulge is not observable is a systematic study of the newly discovered Sagittarius dwarf galaxy. Deep photometry of the galaxy and light curves for newly-discovered variables in Sgr are available from anonymous ftp on sirius.astrouw.edu.pl. Bibliography of OGLE related papers: *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., and Mateo, M.; "The Optical Gravitational Lensing Experiment"; 1992, Acta Astron., 42, 253. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., Preston G., Krzeminski, W. Paczynski, B.; "Microlensing Towards the Galactic Bulge"; 1992, in 'Galactic Bulges', eds. H. Dejonghe and H. Habing (Dordrecht: Kluwer), p. 311 *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., and Mateo, M.; "The Optical Gravitational Lensing Experiment. Color - Magnitude Diagrams of the Galactic Bulge"; 1993, Acta Astron., 43, 69. *** Szymanski, M., and Udalski, A.; "The Optical Gravitational Lensing Experiment. Database of 1992 Results"; 1993, Acta Astron., 43, 91. *** Udalski A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., Preston, G.W., Krzeminski, W., Stanek K. Z. and Paczynski, B.; "The Warsaw - Carnegie - Princeton Optical Gravitational Lens Experiment"; 1993, in "Relativistic Astrophysics and Particle Cosmology", eds. C. W. Akerlof and M. A. Srednicki, Ann. N.Y. Acad. Sci., 688, 626 *** Szymanski M., Udalski, A., Kaluzny, J., Kubiak, M., Mateo, M., Krzeminski, W., Preston, G.W., Paczynski, B.; "The Optical Gravitational Lensing Experiment - First Data"; 1993, in: "Gravitational Lenses in the Universe" - Proceedings of 31st Liege International Astrophysical Colloquium, eds. J. Surdej et al. (Universite de Liege, Institut d'Astrophysique) p. 503. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Krzeminski, W., Mateo, M., Preston, G.W., and Paczynski, B.; "The Optical Gravitational Lensing Experiment. Discovery of the First Candidate Microlensing Event in the Direction of the Galactic Bulge"; 1993, Acta Astron., 43, 289. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. Journal of the 1993 Season Observations."; 1994, Acta Astron., 44, 1. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. The Discovery of Three Further Microlensing Events in the Direction of the Galactic Bulge."; 1994, ApJ Letters, 426, L69. *** Paczynski, B. Stanek, K.Z. Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., and Mateo, M.; "The Distribution of Galactic Disk Stars in Baade's Window."; 1994, Astron. J., 107, 2060. *** Stanek, K.Z., Mateo, M., Udalski, A., Szymanski, M., Kaluzny, J., and Kubiak, M.; "Color-Magnitude Diagram Distribution of the Bulge Red Clump Stars -- Evidence for the Galactic Bar."; 1994, Astrophys. J. Letters, 429, L73. *** Kiraga, M., and Paczynski, B.; "Gravitational Microlensing of the Galactic Bulge Stars."; 1994, Astrophys. J. Letters, 430, L101 *** Udalski, A., Szymanski, M., Stanek, K.Z., Kaluzny, J., Kubiak, M., Mateo, M., Krzeminski W., Paczynski, B., and Venkat, R.; "The Optical Gravitational Lensing Experiment. The Optical Depth to Gravitational Microlensing in the Direction of the Galactic Bulge."; 1994, Acta Astronom., 44, 165. *** Paczynski, B., Stanek, K.Z., Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., and Krzeminski W.; "Are the Ogle Microlenses in the Galactic Bar?" ; 1994, ApJ Letters, 435, L113. *** Udalski, A., Szymanski, M., Mao, S., Di Stefano, R., Kaluzny, J., Kubiak, M., Mateo, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. OGLE #7: Binary Microlens or a New Unusual Variable?"; 1994, ApJ Letters, 436, L103. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., Krzeminski W., and Paczynski, B.; "The Optical Gravitational Lensing Experiment. The Early Warning System: Real Time Microlensing" 1994, Acta Astron., 44, 227. *** Mateo, M., Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. Deep Photometry of the Sagittarius Dwarf Spheroidal Galaxy" 1995, Astron. J., 109, 588.. *** Paczynski, B., Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., Stanek, K. Z., Krzeminski, W., Preston, G. W.: "The Optical Gravitational Lensing Experiment - the Results from 1992 and 1993"; 1994, in: IAU Symposium 169, "Unsolved Problems of the Milky Way", ed. L. Blitz, in press. *** Stanek, K. Z., Mateo, M., Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Krzeminski, W.: "Evidence for the Galactic Bar from Two Color Photometry of the Bulge Red Clump Stars."; 1994, in: IAU Symposium 169, "Unsolved Problems of the Milky Way", ed. L. Blitz, in press. *** Szymanski, M., Udalski, A., Kaluzny, J., Kubiak, M., Krzeminski W., Mateo, M. : "The Optical Gravitational Lensing Experiment. Follow-up Observations of the MACHO Microlensing Event in the Galactic Bulge" 1994, Acta Astron., 44, 387. *** Mateo, M., Kubiak M., Szymanski, M., Kaluzny, J., Krzeminski W., and Udalski, A.; "The Optical Gravitational Lensing Experiment. Variable Stars in the Sagittarius Dwarf Spheroidal Galaxy" 1994, Astron. J. in press. *** Udalski, A., Kubiak, M., Szymanski, M., Kaluzny, J., Mateo, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. The Catalog of Periodic Variable Stars in the Galactic Bulge. I. Periodic Variables in the Center of The Baade's Window" 1994, Acta Astron., 44, 317. *** Kaluzny, J., Kubiak, M., Szymanski, M., Udalski, A., Krzeminski, W., and Mateo, M.; "The Optical Gravitational Lensing Experiment. Variable Stars in the Sculptor Dwarf Spheroidal Galaxy" 1995, Astron. Astrophys. Suppl., in press. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. The Catalog of Periodic Variable Stars in the Galactic Bulge. I. Periodic Variables in Four Baade's Window Fields: BW1, BW2, BW3 and BW4" 1994, Acta Astron., 45, 1. *** Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., Mateo, M., and Krzeminski W.; "The Optical Gravitational Lensing Experiment. Journal of the 1994 Season Observations." 1995, Acta Astron., 45, 237.